Coolant Loss, Bubbles and a Customer Who Wants Stop-Leak
A 1.6 L petrol car with 92,000 km comes in with the complaint that the coolant level drops about 0.5 L every 300 km, with no puddle under the car. The owner reports sweet-smelling white smoke for the first minute after a cold start, the heater blowing cold at idle, and the temperature gauge climbing in stationary traffic before dropping once the car is moving. The expansion bottle has a dark oily film. With the engine idling and the radiator cap off, bubbles appear steadily and increase when the engine is revved. Compression, dry, cylinders 1–4: 12.6, 12.4, 9.8 and 12.5 bar (specification 12.0–14.0 bar, minimum 10.5 bar, maximum difference between cylinders 1.0 bar). The wet test on cylinder 3 raises it to 10.1 bar. Cylinder leak-down on No.3 shows 35 % loss with bubbles rising in the coolant. The customer wants the cheapest possible fix and has been told by a friend that a bottle of stop-leak will do.
1. Interpret the test results: what does each test confirm, and which faults does the evidence rule out?
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What the evidence shows
- Coolant loss with no external leak, oily film and steady bubbles that increase with revs point to combustion gas being pushed into the cooling system. A chemical block test confirming CO₂ would nail it.
- Compression. No.3 at 9.8 bar is below the 10.5 bar minimum and 2.6 bar below the other three, which also exceeds the 1.0 bar spread limit. That is a sealing fault on No.3.
- Wet test. A rise of only 0.3 bar (to 10.1) is small. A ring fault normally lifts the reading by well over 0.5 bar, so the leak is not mainly past the rings; it points to the head gasket, a valve or the head casting.
- Leak-down. 35 % loss with air bubbling in the coolant localises the leak path from cylinder 3 into a water jacket - the definitive finding here.
What is ruled out or still open
- External leaks: pressure-test the system cold and check the water pump weep hole, hoses, radiator tanks, heater matrix and core plugs before stripping anything.
- An EGR cooler or oil cooler, if fitted, can leak internally and mimic a gasket. Bypass or pressure-test it.
- The cold heater and gauge swing are consistent with air locks and low coolant; a stuck thermostat may be a secondary fault, not the cause.
2. What findings during strip-down would make you suspect a cracked head or block rather than only the gasket, and what checks are needed before refitting?
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Suspect a crack, not just a gasket, if:
- The fire ring is intact and there is no obvious gasket failure path, yet coolant still crosses.
- A crack is visible between the valve seats or from a seat to a port, or the head fails a pressure/crack test.
- Coolant appears in the sump or the oil turns to emulsion, or the leak only appears when hot.
- The block deck shows pitting, a crack or a recessed fire-ring area, or a liner has dropped.
Checks before refitting
- Check head flatness with a straight edge and feeler gauge along several lines and diagonals; skim or replace if out of tolerance, and remember that skimming an overhead-cam head affects cam bearing alignment and compression ratio.
- Pressure-test and crack-test the head; check valve sealing and stem seals while it is off.
- Check the block deck for flatness and pitting around the fire rings, and clean the bolt holes of oil and coolant and blow them out - trapped fluid hydraulically cracks the block when the bolt is torqued.
- Measure the head bolts; torque-to-yield bolts are one-use and must be replaced, then torqued in sequence to angle.
- Replace the gasket, thermostat, timing belt and tensioner while accessible, flush the cooling system, refill with the specified coolant, change oil and filter, and pressure-test hot and cold afterwards.
3. The customer insists on a bottle of stop-leak. How do you handle the conversation and what goes into the estimate?
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The conversation Acknowledge the cost concern first, then be specific rather than preachy: a 35 % leak-down into the coolant is not a pinhole radiator weep. Stop-leak is designed for small external leaks; it can plug the heater matrix and does not restore cylinder sealing. A 9.8 bar cylinder with coolant entering it will get worse, and overheating a warped head turns a gasket job into a head or engine replacement. Show the customer the bubbles and the leak-down reading - evidence persuades better than opinion.
The estimate Quote in two stages: a fixed price for strip, inspection and report with photographs, then the repair priced once the head is measured and crack-tested. Do not quote a final figure for an unseen head. Include gasket set, head bolts, coolant, oil and filter, thermostat, and the timing belt and tensioner if due at 92,000 km. Note the possible extra of a head skim or exchange head.
Boundaries If the customer still chooses stop-leak, record the advice given and the refusal on the job card and invoice and have it acknowledged. Do not add stop-leak and hand the car back as repaired; the fault is documented as outstanding. Never fit a gasket without checking head flatness, because a repeat failure will be pinned on your work.